1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
The recent years have seen a marked reduction in size and weight of portable electric devices. Also, the increase of their functions pushes up their power consumption. This has led to a strong demand for further improvements of secondary lithium batteries which serve as a power source of those devices, in terms of weight reduction and capacity increase.
As a measure to meet such a demand, the use of silicon as an electrode material has been recently proposed which shows superior charge/discharge capacity, both per unit mass and unit volume, compared to a carbon electrode.
In conventional secondary lithium batteries using a silicon thin film as a negative active material, an electrode has been proposed including a thin film of active material which is divided into columns by gaps formed therein and extending in its thickness direction. The provision of such a columnar structure lessens a stress engendered due to expansion and shrinkage of the active material, prevents separation of the active material from a current collector and thereby improves charge-discharge cycle characteristics.
It is however known that, in the negative electrode using such a silicon thin film, the active material changes in properties and increases in porosity with repeated charge-discharge cycling. As will be described later, the inventors of this application have discovered that such a change in properties of the active material results from oxidation of silicon during operation of a battery. The present invention is based on such a knowledge of the inventors.
Japanese Patent Laying-Open No. 2004-349079 proposes the use of a pH control agent in order to retard oxidation of silicon in the manufacture of an electrode. However, in Japanese Patent Laying-Open No. 2004-349079, neither disclosure nor suggestion is provided as to oxidation of silicon during charge-discharge cycles.
Japanese Patent Laying-Open No. 2004-6188 describes that incorporation of a saturated dicarboxylic acid in an interior portion of a negative electrode improves charge-discharge cycle characteristics. Japanese Patent Laying-Open No. 2004-335379 describes that incorporation of an organic acid in an interior portion of a negative electrode improves cycle characteristics. However, neither of these prior art references provides disclosure as to improvement of charge-discharge cycle characteristics by retarding oxidation of silicon.